Hot dip tinning a high carbon ferrous metal



United States Patent m U.S. Cl. 204-38 16 Claims ABSTRACT OF THEDISCLOSURE Hot dip tinning a high carbon ferrous metal such as cast,grey, malleable or wrought iron by first cleaning from the metal anysoil which might prevent electroplating on it, then electroplating on itan electrodeposit of iron from an acid iron-plating bath containing anorganic sequestering agent and an iron compound.

This application is a continuation-in-part of my copending applicationSer. No. 172,118 filed Feb. 9, 1962 and now abandoned.

This invention concerns the hot tin dipping, i.e. hot tinning, ofcoarse-surfaced ordinarily relatively high carbon content ferrous metalsand alloys, such as cast iron as grey cast iron, malleable iron, andwrought iron, the hot tinning of which heretofore could not beaccomplished readily and at best could be carried out only byundesirable methods involving considerable hazard and expense andyielding only erratic results.

More specifically the invention is that of hot tinning a coarse-surfacedferrous metal by initially cleaning it of any soil which could preventadherence of a fresh metal electrodeposit on it; electroplating on it afirmly adherent at least masking electrodeposit of nascent iron from aniron electroplating bath having a pH from 7 to about 3 and containing asequestering agent, and thereafter hot tinning it in the usual Way used,for example, on cold rolled steel.

The high carbon content ferrous metals such as cast iron as grey castiron, malleable iron, and wrought iron ordinarily, that is to say whenclean (e.g. free of molding sand or scale) and before being surfaceground or polished, have a coarse surface, apparently because of theircoarse grain structure as distinguished from that of the low carbonsteels which are easily rolled and shaped and still present asubstantially overall flat, smooth surface.

Accordingly, the said coarse-surfaced high carbon content ferrous metalsand alloys which include among them also those having high siliconcontent such as the acidresistant ferrous metals as the one long knownby its trademark Duriron, conveniently can be called ferrous metalsordinarily having a coarse surface or singly an ordinarilycoarse-surfaced ferrous metal. The Duriron alloy is a ferrous cast alloycontaining, in addition to iron, 14.5% silicon, 0.85% carbon, and 0.65%manganese.

Heretofore, considerable difficulty was encountered in trying toaccomplish regularly satisfactory practical hottinning of an ordinarilycoarse-surfaced ferrous metal. Generally, it has been highly difficult,and for the most part possible only in a limited way and under specialconditions, dependably to hot tin coat an ordinarily coarsesurfacedferrous metal, especially cast iron such as grey cast iron.

British Pat. 11,698 of 1906 to John Swain describes coating cleaned castiron articles with a layer of pure iron from an electrolytic bathprepared from iron or steel turnings, hydrochloric acid and lye,preferably using 2 pounds 3,547,787 Patented Dec. 15, 1970 of turningsto 4 pints of hydrochloric acid of 1.14 specific gravity, and half anounce of alkali preferably caustic soda. The latter is said to combinewith the hydrochloric acid to give sodium chloride, and that common saltmay be used instead.

That is a very confused direction giving a bath apparently uncertain asto operation because 4 pints of 1.14 specific gravity hydrochloric acidprovides only 1.3 pounds of hydrogen chloride. That amount is only halfof the equivalent needed for 2 pounds of iron or steel turnings andthereby wholly inadequate when one part of ferrous chloride dissolves inone part of water only in the presence of hydrochloric acid. That mayexplain why no use appears to have been made of that. procedure, butinstead other methods were tried.

Thus, to a limited extent hot tinning has been carried out with somevery few ordinarily coarse-surfaced ferrous metals, for example, byfirst subjecting the such ferrous metal article to a specialpro-treatment in a molten bath of one of a few available proprietaryinorganic cleaning and descaling compositions. This involvespreliminarily preparing the surfaces of such ferrous metal products forthe hot tin dipping operation by immersing them, for example, in acatalyzed molten salt reduction bath (having a melting point of about500 F.) and at an operating range between about 850-950 F'., and passinga current through the molten bath.

Such a procedure is overly costly due to the energy required to maintainthe bath molten. It also is highly hazardous to the operators workingabout such bath, for example, from dragout on the articles leaving suchhot bath. In addition the results are inadequate for there appears to bea too frequent lack of uniformity in the coating, so that the percentageof rejects is undesirably too high for general practical operation.

The foregoing disadvantages and others are overcome by the process ofthe invention which is a significantly less costly, relatively muchquicker, and considerably safer method of hot tinning. Also, it providesa resulting more regularly and uniformly dependably tin coated product.

Considered broadly, the process of the invention is that of applying ahot tin coating to an ordinarily coarsesurfaced ferrous metal, byremoving from the surface of said metal any soil (including rust) whichinitially should be removed, electroplating on the clean coarse-surfacedferrous metal in an iron electroplating bath having a pH of from 7 toabout 3 and in the presence of a sutficiently effective quantity of asequestering agent a firmly adhering, substantially continuous, at leastmasking iron electrodeposit; removing the thus electroplated ferrousmetal from the plating bath and any plating solution adhering to theplated metal; and thereafter dipping the thus plated ferrous metal, withor without intermediate immersion in a tinning flux bath, into themolten tin bath to apply the tin coating to that iron electroplatedferrous metal. The masking electrodeposit is essentially purecarbon-free iron.

In describing the electrodeposit of iron as firmly adhering,substantially continuous, at least masking the expression firmlyadhering means that the deposit adheres to the ferrous base metalwithout flaking and is non-peeling from that ferrous base metal.

The expression at least masking means that the deposit is thick enoughat least to mask the generally overall finely spotted or matted surfaceof the ordinary coarse surface of the high carbon content ferrous metal,to the extent that the finely spotted or matted surface is replaced by asubstantially uniform, continuous silvery-grey clean iron appearance.

The method of the invention can be carried out by depositing the firmlyadherent, substantially continuous, at least masking electrodeposit ofiron on the coarsesurfaced ferrous metal from an iron plating bathhaving a pH from 7 to about 3 and containing a sufficiently effectiveconcentration of a sequestering agent. Obviously, before starting theelectrodeposition, any rust or other soil which could prevent uniformadherence of the electrodeposit, should be removed from the surface tobe hot tin coated.

In many cases wherein the residual molding sand or oil or grease, orrust are not extraordinarily excessive, advantageously such soil can beremoved in the same bath in which the iron electrodeposition is to becarried out, especially if there is no oil or grease or possibly only aninsignificant amount of them.

Effective as such iron plating bath for the firmly adheringelectrodeposit of iron is any of a wide variety of aqueous iron platingbaths having a pH from 7 to about 3 and dissolved therein variouseffective amounts of one or more sequestering agents which form aWater-soluble chelate with iron, whether ferrous or ferric.

Particularly effective sequestering agents are the various monohydroxyor polyhydroxy, monoor polycarboxy lower aliphatic acids having from twothrough seven carbon atoms such as gluconic acid, citric acid, tartaricacid, glucoheptonic acid, and its isomers galactoheptonic acid,fructoheptonic acid, and the mixed hexahydroxyheptoic acids, andsaccharic acid, or an amino, polyhydroxy lower aliphatic acid such as3-an1ino-2,4,5,6,7- pentahydroxyheptoic acid, or other sugar acid, aswell as the alkali metal and ammonium salts of any of those acids andthe alkaline earth (including magnesium with them; and at least forferric iron) salts of any of those polycarboxylic acids.

Any of these various six or seven carbon atom polyhydroxy acids or anyof the sugar acids can be admixed with one another or with any of theother sequestering agents, and advantageously with from about one-thirdto three times its quantity of a hexitol such as sorbitol or mannitol.

The sequestering agents include also the polyalkylene polyaminepolyacetic acid compounds and their monoand divalent metal salts, forexample, diethylenetriamine pentaacetic acid and any of its alkali metaland ammonium salts or even any of its alkaline earth salts (at least forferric iron) as its calcium or magnesium salts, and any of themono-hydroxyethyl-tetra-carboxymethyl diethylenetriamines ordihydroxyethyl-tricarboxymethyl diethylenetriamines, and any of thecorresponding same salts of any of them, as well as any of the free acidand salt form sequestering agents disclosed in US. Letters Patent2,831,885; 2,848,469; 2,859,104 and 2,906,762.

While individual plating baths can be prepared using any one of theforegoing and other effective sequestering agents along with a suitableamount of alkali metal hydroxide to give an effective pH value notexceeding 7, more than one of any of the applicable sequestering agentscan be used. There can be included various amounts of ethylenediaminetetraacetic acid or of any of its monoto tetra-alkali metal or ammoniumsalts as well as any of its alkaline earth metal salts (includingmagnesium among them), and generally to the extent of no more than aboutone-half the amount of the other sequestering agent, or of a loweralkanolamine such as mono-, di-, or triethanolamine and likepropanolamines.

Effective aqueous acid iron plating baths can be prepared with one ormore of the free acid sequestering agents from those mentioned aboveand/or with some relatively neutral water-soluble salts of any of them,to provide a bath having such pH below 7, between which the specificsequestering agent or agents used can complex with iron to form achelate. Such acid baths can include also various amounts of an alkalimetal or ammonium dihydrogen phosphate such as monosodium ormonopotassium dihydrogen phosphate.

For more effective iron plating with an acid bath, its pH can be in therange under 7 to about pH 3.5. A bath can be prepared with pH as low as3.0 to 3.2, at which a suitable iron electrodeposit can be obtained.However, at such low pH level with many of the acid baths showrelatively poor throwing power and the plating rate may be too low. Theminimum for generally practical operation should be at least about pH3.5, and beneficially at least about 4.0.

Ordinarily after some use of an aqueous acid bath, especially startingat a low pH as near 3.0, its pH increases as a result of dissolution ofiron from the ironcontaining anode. That can enable preparing, ifnecessary, a bath with an initial pH of as little as say 3.2 if theworking load or other conditions do not make it unsatisfactory tooperate the bath through what may be called a breaking in period whilethe pH rises to 3.5 or 3.6 or so.

When the preliminary cleaning is not going to be conducted in theplating bath or when the amount of rust to be removed is low, theaqueous plating bath should contain in solution at least a suflicientamount of a compatible iron salt or chelate, which is soluble at the pHof the plating bath to avoid exhausting its iron if no ironbearing anodeis being used. It is advantageous, however, to use some suitableiron-bearing anode. Thus generally, the starting composition of the bathneed contain only very little of a water-soluble iron compound such asan iron salt or chelate when the bath will be used to clean a sufficientamount of rust from the cast iron articles which are to be given theiron electrodeposit, or ironbearing anodes are to be used.

To provide the initial iron content of the bath when iron-bearing anodesare used, it can contain a relatively small amount such as aboutone-tenth percent of such water-soluble iron salt as a ferrous or ferricsalt soluble at the pH of the bath, such as ferrous or ferric sulfate,chloride, or nitrate, or as thus far noted to be advantageous ferricacetate, as well as any of the iron chelates of any of the applicablesequestering agents and soluble at the pH of the bath at least to theextent sufficient to enable electrodeposition of iron to be initiatedtherein.

In some cases, initial content of such iron salt or chelate can be quitesmall and at times avoided when the bath is to be used for thepreliminary cleaning of the cast iron or other ordinarilycoarse-surfaced ferrous metal which is to receive the ironelectrodeposit. That is so because such preliminary cleaning, carriedout advantageously by peri odic reverse current procedure, results inproviding an adequate initial amount of dissolved iron in the bathsufiicient to enable electrodeposition of iron on the cathode toprogress by continued dissolution of iron from the particulariron-containing anode used to enable depositing the necessary firmlyadhering electrodeposit of iron on the originally coarse-surfacedferrous metal cathode.

For regularly dependable deposition of iron, the total dissolved solidsin the bath can range from about one to about four pounds per gallon(i.e. about 115 to about 450 grams per liter). A generally goodpractical concentration, bearing in mind such factors as conductivity,plating rate, and dragout is in the neighborhood of about tWo to about2.5 pounds per gallon. However, for higher conductivity with certainsolutions (e.g. sodium gluconate without any other added salts), it ismore desirable to work with solutions of at least two pounds and nearerabout four pounds per gallon.

The concentration of the sequestering agent, whether one or a mixture ofthem, can vary widely, generally from about two to about one hundredpercent of the total solids content, and preferably from about five toabout ninetyfive percent of it, depending on providing the required pHvalue or range.

Grey cast iron, or black iron or other cast iron is very satisfactoryfor anodes of ferrous material to replenish iron to the bath as it isplated out, to provide a consistently uniform masking ironelectrodeposit on the initially ordinarily coarse-surfaced ferrous metalcathode. Elec trolytic iron anodes also are suitable. At times even coldrolled steel anodes can be used. To avoid interference with the qualityof the iron deposit by suspended carbon particles released from a highcarbon content ferrous metal anode over continued use, it is desirableto enclose such anodes in Orlon or other suitable anode bags, aspreferable over periodic or continuous filtration of the bath.

Consideration should be given to the relationship of anode area to thatof the articles being plated and thus serving as cathodes. Generally, itis advisable that the anode area be significantly greater than that ofthe part to be plated, and even up to double its area particularly ifthe cathode part has deep hollows.

The bath may be operated over a wide temperature range, even as low asambient (i.e. room) temperature,

but at such level the plating rate is very slow (i.e. at about 80 F),and the voltage needed for suitable current density is excessive, beingfrom 12 to 15 volts or even more. A presently indicated most practicaltemperature range is from about 140 to about 180 F., although there isno discernible difference in the adhesion and generally desirablecharacter of the iron electrodeposit even at the lower temperatures.Where conditions permit, very satisfactory practical results occur at ashigh as 190 F. and are obtained even at 200 F. and can be obtained alsoat possibly even a higher point. It appears generally advisable, ofcourse, to work safely below the baths boiling point.

Current density, for generally good results, should range from about toabout 80 amperes per square foot, and under many conditions can be ashigh as 100 amperes per square foot. However, for cathode articleshaving sharp points or projections, it may be advisable to operatesomewhat under 80 amperes per square foot to avoid burning. Ordinarilythe lower the temperature, the higher should be the current density.

Thus, the maximum current density for any particular bath should be justunder that at which the electrodeposit would begin to show signs ofburning. However, the current density generally would have to be wellover 100 amperes per square foot before any indication of burning orother undesirable injury can occur to the iron electrodeposit on theoriginally coarse-surfaced ferrous metal article cathode, or a flaky(and thus undesirable) electrodeposit can be produced.

Electrodeposition time has to vary with the character of the surface tobe electroplated, bath composition, temperature, current density, andany other plating condition. With some combinations of conditions,possibly as little as about seven minutes or so may be adequate, and inothers more than that and possibly up to about fifteen, or even an abouttwenty-five minute or more plating cycle may be needed; the rougher thesurface, the lower the temperature, the longer the time requiredordinarily. Also the heavier or bulkier the work piece, the longer thedeposit time.

The method of the invention is operable readily in quantity productionscale. Quite often the bath used for the plating step of the method ofthe invention can be used for the preliminary treatment to remove theaverage ordinary amounts of soil and rust encountered on the general runof articles which will need to be iron plated in the bath. As alreadyindicated, such soil can be removed by subjecting the articles topreliminary electrolytic treatment, including periodic reverse current,for a time sufficient to remove the soil and rust, That will depend onthe type and extent of soil and rust, the bath, and the treatment.

For some combinations of these conditions, including mild soil and/orrust, two or three minutes of periodic reverse current treatment may beadequate. Slightly heavy soil and rust, possibly may need from about tento al most fifteen minutes. For heavy soil and/or rust conditions, evenup to about thirty minutes or so may be required.

For some soils, perhaps more so with oil and grease, it can help toinclude a small percentage, generally under one-half percent andpossibly more often about half of that or less, of a syntheticdetergent, nonionic or anionic and at times even cationic, or a mixtureof any of them, as specific conditions may dictate.

As stated earlier above, the iron electrodeposit needs to be sufiicientto mask the initial ordinarily coarse, generally grainy surface of thecathode article. Because of the relatively rough and irregular surfaceof even the initial coarse-surfaced ferrous metal base, in that it isnot fully flat and smooth as in the case of low carbon steel, nospecific numerical minimum electrodeposit thickness can be given foreach dilferent ordinarily coarse-surfaced ferrous metal. However, theelectrodeposit thickness appears to be adequate when the cleaned surfaceis covered with the plated iron to the extent that the plated surfaceappears to be a firmly adherent, overall substantially continuousmasking iron electrodeposit.

Such adherent, overall substantially continuous minimum deposit then issufficient to resist flaking or being destroyed or burned away at thetemperature of the molten tin bath used in the hot tinning operation, astin melts so far below these ferrous metals. Ordinarily, the ironelectrodeposit thickness does not have to be much more than that justdescribed above, even though the thus plated surface then may not beentirely flat. A slightly thicker deposit even below 0.0001 inch couldbe more practical.

However, it is difficult also to set a numerical maximum electrodepositthickness applicable to all surfaces of the various coarse-surfacedferrous metals. While about 0.0001 inch thick might be more than enoughfor most conditions, yet, where particularly needed or desired, it couldbe as much as up to about 0.0002 inch. Generally, there does not appearto be any particular need to plate a deposit that thick or thicker thanit.

While the method of the invention is applicable to hot dip tinning ofany ordinarily coarse-surfaced ferrous metal article, it is applicableparticularly to such tinning of cast iron such as grey cast iron.Accordingly, the in- 7 equally to such tinning of any othercoarse'surfaced high carbon content ferrous metal and even nodular iron.

Cast iron castings of a rectangular prismatic box (25 inches long by 4inches square inside cross-section) open at one side are cleaned ofadhering loose mold sand, in customary manner. They then are hungsuitably spaced from one another from horizontal arms of a cathode rackand immersed, properly spaced from grey iron anodes (of about double thecathode area), in an aqueous iron plating bath held at about 200 F. andcontaining per liter 120 grams of sodium gluconate and 120 grams ofgluconic acid.

Since these cast boxes were dirty and rusted in areas, they weresubjected in this bath. for fifteen minutes to periodic reverse currentof five seconds direct current to the cathode, and ten seconds thereverse, to clean them. Without removing them from this bath, directcurrent (see to deliver amperes per square foot of cathode area) then ispassed from the anodes to these boxes as cathodes to deposit iron onthem for fifteen minutes. The castings then are removed, rinsedadequately with warm water and allowed to air dry. They show acontinuous and uniform electrodeposit of iron firmly adhering to andcompletely masking the original speckle spotted cast grey iron surface.

The boxes on half of the racks then are immersed in the customarytinning flux bath. Then these and the boxes on the other half of theracks are submerged in customary manner and for the usual time in themolten tin bath, and removed and allowed to cool. Both batches of theboxes show uniformly overall adherent dependable tin coatll'lgS.

Any of the various combined plating bath operating conditions in theforegoing more fully described illustrative operation can be changed atleast within the various ranges disclosed herein to be practical. Soalso the same bath at any combination of suitable operating conditionscan be used for cleaning and also to plate a corresponding masking ironelectrodeposit on any other such castings of the same iron or any otherordinarily coarsesurfaced ferrous metal of the type disclosed herein.

As already stated above, the already described firmly adherent, masking,overall substantially continuous electrodeposit of iron can be platedout of any other suitable iron plating bath used in this invention, suchas are illustrated by, but not restricted to, the aqueous iron platingbath compositions containing respectively the following:

Grams per liter Sodium glucoheptonate 120 Gluconic acid 120 Thatquantity of gluconic acid was provided by using 240 grams per liter of acommercially available aqueous solution containing 50% gluconic acid.The bath pH is 3.5.

Grams per liter Citric acid 195 Sodium hydroxide 45 In this bath (of pH3.6) the sodium hydroxide neutralizes part of the citric acid so thatthe composition of the bath actually is about 120 grams each of sodiumcitrate and The pH is 6.6. The pentaacetate here also was used as 34%aqueous solution.

The sodium hydroxide of bath (b) can be replaced by any other hereinindicated applicable alkaline agent, even compatible amine, e.g.diethylamine or a mono-, di-, or triethanolamine or mixtures of them solong as the bath pH still is below 7 and above about 3.2 andbeneficially at least about 3.5.

The sodium citrate resulting from the neutralization in bath (b) can beadded directly as sodium, or other alkali metal or ammonium, citratewithout any noticeable difference, or even by a compatible amine salt ofit as indicated in the preceding paragraph.

The gluconic acid of bath (a) and citric acid of both (b) can bereplaced in whole or part by any quantity of the other of them or of anyother hereinabove disclosed acid sequestering agent which issufficiently soluble in water for the pH of the bath to be within therecently above indicated acid range.

The sodium diethylenetriamine pentaacetate can be replaced in whole orpart by its corresponding salt of any other alkali metal or its ammoniumor other above-indicated amine salt, or by the corresponding tetra-,tri-, di-, or mono-acetate, or even of diethylenetriamine pentaaceticacid itself or by any other neutral or acid sequestering agentsufficiently soluble in water for the pH of the bath to be within theabove disclosed range.

The sodium dihydrogen phosphate can be replaced in part or as a whole byany other alkali metal or ammonium dihydrogen phosphate or even by awater-soluble lower alkyl (with 1-8 carbons) acid phosphate or awatersoluble alkali metal, ammonium, or amine (as above indicated) acidsalt of such alkyl acid phosphate with up to about 18 carbon atoms inthe alkyl chain, and so long as the concentration of any of them keepsthe pH of the bath from 3 to 7.

The throwing power of these acid baths in many instances may not be ashigh as that of an alkaline bath containing water-soluble salts of thecorresponding organic acid sequestering agents. On the other hand,articles iron plated in the acid baths are more readily rinsed becauseof their generally lower viscosity.

The presently indicated preferred acid bath is one containing citricacid, e.g. as in bath (b), and even operated at a low pH about 3.5.Thus, the hereinabove (page 13 line 8 to page 14 line 11) fullydescribed illustrative operation, showing the overall treatment of castiron boxes, provides similarly practical results with plating bath (b)used in place of the bath in the above illustrative description. Thatmore fully described illustrative operation then is to be considered asseparately recorded here in full but with its alkaline bath replaced byacid bath (b).

In addition, that originally included acid bath (b) in that completeoperation can be replaced by any other bath respectively specificallyidentified in any of the foregoing specific baths or any above explainedpossible modifications of them. Moreover, the method of the inventionsimilarly can be carried out even in a bath of pH 7 just as it can be inany below that.

However, where it is possible to conduct the electrolytic cleaning inthe same bath from which the iron deposit is to be plated out, the bathshould be low enough on acid pH to enable a practical rate of suchcleaning to occur.

It is mentioned above to include a water-soluble iron salt in a platingbath to facilitate initiating iron electrodeposition in it especiallyWhen that same bath is not used initially to clean the article to beplated. In such case, it is possible to include in an acid bath an ironsalt, e.g. ferrous or ferric phophate, which while ordinarily insolubleor of limited solubility in water will dissolve in the acid bath becausethe sequestering agent ingredient of the bath will chelate the iron ofthat salt.

Any water-soluble iron salt or chelate (e.g. ferrous gluconate) for theforegoing purpose advantageously can be included in a dry mix containingone or more such salt or chelate or other iron (ferrous or ferric) saltdirectly soluble or by chelation in the plating bath at its specific pH,along with the selected sequestering agent or agents, and also anyalkaline or acid substance Which may be needed to adjust the bath to therequired pH after the mix is dissolved in water. If cost were not adeterrent, the soluble form of iron and the required chelating agent oragents could be combined and included in the mix as the iron chelateequivalent of the quantity of sequestering agent content required.

The separate iron salt or chelate then could be included in any such drymix in an amount to provide a minimum of, say, from about 0.02 to about0.1 percent of iron in the aqueous plating bath. That will provide about0.2 to about one gram of iron per liter of such aqueous bath. The drymix then can contain enough of the iron salt or chelate to provide aminimum of, say, from about 0.2 to about 1.25 grams of iron per hundredgrams of dry mix to be added at from about to 600 grams (of mix) perliter of water.

More watersoluble iron salt can be included in such mix, up to a maximumwhich conveniently can be set at the amount of iron required to form theiron chelate with all of the sequestering agent content of the mix.However, it is advisable to keep the iron content of the mix below itschelate equivalent of all of its sequestering agent content.

The dry mixes are illustrated by, but not restricted to, the followingcompositions:

Grams Sodium glucoheptonate 120 Gluconic acid 120 Ferrous gluconate 10This mix dissolved in a liter of water gives a pH about that of bath (a)above.

Grams Citric acid 120 Sodium citrate 120 Ferric citrate 10 This mixdissolved in a liter of water gives a pH like that of bath (b) above.

The ferrous gluconate or ferric citrate in any of these dry mixes can beincreased or decreased within the hereinabove indicated range of contentof iron in the dry mixes, or can be replaced in part or as a whole bythe iron equivalent amount of the other of them or of any other ferrousor ferric chelate of any other chelating agent, such as those shown inany of the patents identified above, or by any ferrous or ferric salt orsalts soluble in the bath, e.g. ferric acetate.

Any of the sodium glucoheptonate or citrate, or citric or gluconic acid,in any of these dry mixes can be replaced in part or as a whole by acorresponding amount of any other alkali metal or ammonium salt or otherhereinabove indicated salt of any of them or of any other applicablesequestering agent available in solid discrete particle form, with orwithout any minor amount of sodium or other alkali metal carbonate whichmay need to be added to prevent caking. All such dry mixes and those of'(d) and (e) are embraced herein as a part of this invention.

The method of the invention works also with the low carbon ferrousmetals such as the low carbon steels which can be cold rolled and shapedby such operations as forging and spinning. However, ordinarily such lowcarbon ferrous metals readily can be hot tipped.

Thus, the method of the invention is beneficial primarily with thoseferrous metals, including their alloys, with which some difficulty ordisadvantage is met in attempts to hot tin coat them so that theygenerally cannot be hot tin coated readily or else require some specialpreliminary treatment more difficult, hazardous, and/r costly thanmerely preliminary electrodeposition of iron.

Herein and in the appended claims, the expression ferrous metal is to bebroadly construed as including the ordinary commercial forms of thevarious irons as well as its common alloys composed highly predominatelyof iron.

While the invention has been explained more extensively by detaileddescription of certain specific illustrative embodiments of it, it isunderstood that various modifications and substitutions can be made inany of the thus described embodiments within the scope of the appendedclaims which are intended also to include equivalents of any suchembodiments.

What is claimed is:

1. The method of hot dip tinning a ferrous metal selected from the classconsisting of cast iron, grey iron, malleable iron, mottled iron, whiteiron, wrought iron, and any other ordinarily coarse-surfaced ferrousmetal which ordinarily can be so tinned only in a limited way and underspecially different conditions, which method comprises (a) initiallycleaning from the surface of such ferrous metal any soil which couldprevent electroplating thereon an adherent electrodeposit of iron;

(b) electroplating on said surface a firmly adherent substantiallycontinuous, at least masking electrodeposit of iron while said cleanedferrous metal is a cathode immersed in an aqueous iron-plating bathhaving (i) a pH from seven to about 3.0 and (ii) dissolved therein anorganic sequestering agent in an amount substantially suificient foriron in any iron compound in the bath to be present as the iron 10chelate of said sequestering agent, and for a time sufficient for saidelectrodeposit to be plated on said metal;

(c) removing the thus iron-plated ferrous metal from said bath and anyplating solution adhering to said electrodeposit; and

(d) thereafter contacting said iron-plated surface of the ferrous metalwith molten tin thereby applying over said surface a continuous coatingof tin, and allowing said tin coating to cool to solidify.

2. The method as claimed in claim 1, wherein after removing any platingsolution from the plated ferrous metal removed from the plating bath andbefore contacting the iron plated surface with the molten tin, theplated ferrous metal surface is contacted with tinning flux.

3. The method as claimed in claim 1, wherein the sequestering agent ishydroxy lower aliphatic, soluble in the bath, and includes at least onegroup COOR wherein R is a cation which can be replaced by iron atWhichever valence it exists in the bath and for it to form a chelateWith the ligand portion of said sequestering agent.

4. The method as claimed in claim 3, wherein the sequestering agent ispolyhydroxy.

5. The method as claimed in claim 4, wherein the sequestering agentcontent of the bath has 6 carbon atoms in a straight chain and ahydroxyl group attached to each of the 5 carbons other than in the groupCOOR.

6. The method as claimed in claim 5, wherein the sequestering agent isan alkali metal gluconate.

7. The method as claimed in claim 4, wherein the pH of the bath is fromseven to about 3.5.

8. The method as claimed in claim 7, wherein the bath contains alsoanother sequestering agent which is a hydroxy lower aliphatic carboxylicacid having from two to seven carbon atoms, from one to six hydroxylgroups, from one to three carboxyl groups, and is only hydroxy andcarboxy substituted.

9. The method as claimed in claim 3, wherein the sequestering agentcontent of the bath comprises H2O. COOR HO-C. COO'R H2O. COOR wherein Ris a cation which can be replaced by iron at whichever valence it existsin the bath and for it to form a chelate with the ligand portion of saidsequestering agent.

10. The method as claimed in claim 9, wherein the pH of the bath is fromseven to about 3 .2.

11. The method as claimed in claim 10, wherein the sequestering agentcontent consists essentially of citric acid and an alkali metal citrate.

12. The method as claimed in claim 3, wherein prior to direct currentdeposition of iron on the ferrous metal, soil is removed from it bysubjecting it to the effect of periodic reverse current in the platingbath for a time sufiicient to remove any soil from its surface; and thensubjecting the thus cleaned ferrous metal as a cathode in the same bathto direct current to electroplate on it said electrodeposit of iron.

13. The method as claimed in claim 12, wherein the pH of the bath isbelow seven.

14. The method as claimed in claim 13, wherein the plating bath containsabout grams per liter each of gluconic acid and sodium glucoheptonate.

15. The method as claimed in claim 3, wherein the sequestering agentcontent ofi the bath comprises and R is defined as in claim 3.

1 1 1 2 16. The method as claimed in claim 15, wherein atleast OTHERREFERENCES half of the sequesterlng agent content of the bath 1s anGraham, Electroplating Engineering Handbook; 1955 alkali metalglucoheptonate. Reinhold Publ 128450 References Cited 5 JOHN H. MACK,Primary Examiner UNITED STATES PATENTS T. TUFARIELLO, Assistant Examiner2,714,089 7/1955 Meyer 2o4-4s 3,380,151 4/1968 Parsons 29488 U.S.C1.X.R.

FOREIGN PATENTS 10 20432, 48

11,698 3/1907 Great Britain 20448

